Subject: Re: [PATCH net-next v1] net: sched: cls_u32: Avoid memcpy() false-positive warning in u32_init_knode()

From: "Jiayuan Chen" <[email-masked]>
Date: Thu, 05 Mar 2026 08:09:37 +0000

March 5, 2026 at 15:33, "Jiayuan Chen" <[email-masked] mailto:[email-masked]?to=%22Jiayuan%20Chen%22%20%3Cjiayuan.chen%40linux.dev%3E > wrote:

> 
> From: Jiayuan Chen <[email-masked]>
> 
> Syzbot reported a warning in u32_init_knode() [1].
> 
> Similar to commit 7cba18332e36 ("net: sched: cls_u32: Avoid memcpy()
> false-positive warning") which addressed the same issue in u32_change(),
> use unsafe_memcpy() in u32_init_knode() to work around the compiler's
> inability to see into composite flexible array structs.
> 
> This silences the false-positive reported by syzbot:
> 
>  memcpy: detected field-spanning write (size 32) of single field
>  "&new->sel" at net/sched/cls_u32.c:855 (size 16)
> 
> Since the memory is correctly allocated with kzalloc_flex() using
> s->nkeys, this is purely a false positive and does not need a Fixes tag.
> 
> [1] https://syzkaller.appspot.com/bug?extid=d5ace703ed883df56e42
> 
> Reported-by: [email-masked]
> Closes: https://lore.kernel.org/all/[email-masked]/T/
> Signed-off-by: Jiayuan Chen <[email-masked]>
> Made-with: Cursor
> ---
>  net/sched/cls_u32.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
> index 9241c025aa74..8f30cc82181d 100644
> --- a/net/sched/cls_u32.c
> +++ b/net/sched/cls_u32.c
> @@ -852,7 +852,10 @@ static struct tc_u_knode *u32_init_knode(struct net *net, struct tcf_proto *tp,
>  /* Similarly success statistics must be moved as pointers */
>  new->pcpu_success = n->pcpu_success;
>  #endif
> - memcpy(&new->sel, s, struct_size(s, keys, s->nkeys));
> + unsafe_memcpy(&new->sel, s, struct_size(s, keys, s->nkeys),
> + /* A composite flex-array structure destination,
> + * which was correctly sized with kzalloc_flex(),
> + * above. */);

I'm thinking whether it's more appropriate:

new->sel.hdr = s->hdr;
memcpy(new->sel.keys, s->keys, flex_array_size(s, keys, s->nkeys));

>  if (tcf_exts_init(&new->exts, net, TCA_U32_ACT, TCA_U32_POLICE)) {
>  kfree(new);
> -- 
> 2.43.0
>